The present invention relates generally to data link control methods, and more particularly to a data link control method for controlling the receiver and transmitter so that a data transmission time can be shortened even though the receiver has a relatively long minimum recording time.
A well-known facsimile apparatus (abbreviated FAX hereinafter) comprises a scanner, an encoder/decoder, a memory, and a plotter. The scanner scans every line of an image on a document to generate a image signal corresponding to the image. The encoder/decoder encodes the image signal for every line into image data and decodes the image data for every line into the image signal. Thus, the FAX transmits and/or receives an image data corresponding to a predetermined image for every line. The image data comprises, as shown in FIG. 1A and 1B, an end-of-line signal (abbreviated EOL hereinafter) representing an end of a line, code data representing the predetermined image for one line, and a fill-bit representing "0". Hereupon, Na denotes a bit number of the image data, Nc denotes the sum of the bit number of the EOL and the code data, and Nf denotes that of the fill-bit. The fill-bit is added when Nc is less than Na which is determined by the minimum recording time and the data transmission speed so that the scanning operation of the transmitter can be synchronized with the plotting operation of the receiver. The image data transmission time can be shortened by accelerating the data transmission speed or by lessening the bit number of the image data. Since the highest data transmission speed is 9600 bps determined by a V.29 MODEM according to Comite Consultatif International Telegraphique et Telephonique (abbreviated CCITT hereinafter), the bit number of the image data should be lessened by deleting the fill-bit. That is, the minimum recording time for 1 line of a receiver is required to be less than the scanning time for 1 line of a transmitter. But a thermal printer which is generally equipped with the conventional FAX and operates at a relatively low speed cannot satisfy the request. Accordingly, a skip mode has been proposed to cope with the request. In this mode, as shown in FIG. 1C, the fill-bit is added to Nc comprising the EOL and blank line so that the sum of bit number of the EOL, the blank line, and the fill-bit can correspond to 1/2 Na. The terms "blank line", as used hereinafter, means a line on which the image does not exist. The data amount of the blank line is reduced to a ratio of a normal speed of the recording paper to a feeding speed thereof. In this case, since, when the receiver detects the blank line, it feeds the recording paper at the feeding speed which is twice as fast as the normal speed, the data amount of the blank lines is reduced to half thereof.
Thus, since the data amount of the blank line is limited to half thereof and the feeding speed of the blank line during plotting is set to be twice as fast as the normal speed, the plotting operation of the receiver can be synchronized with the scanning operation of the transmitter. For example, when the minimum recording time is assumed to be 10 ms and the data transmission speed 9600 bps, the minimum confirmation bit number NA becomes 96 bits and half thereof becomes 48 bits. In addition, when a scanning width is A4 and a pixel density is 8 dot/mm, the pixel number for 1 line becomes 1728. Therefore the code data representing the blank line becomes 17 bits in MH encoding. Since the bit number of the EOL is 12 bits, the sum of the bit number of the EOL and the blank line code becomes 29 bits, which is less than 48 bits, so that the fill-bit having 19 bits is added thereto. Thus, the data amount of the blank line which is a large amount of the normally transmitted image data can be surprisingly deleted, so that the image data transmission time for 1 page can be shortened.
However, the conventional data link control method has the following disadvantage when the bit number of the blank line is too small for Na. That is, for example, when the minimum recording time is assumed to be 20 ms and the data transmission speed 9600 bps, the minimum confirmation bit number NA becomes 192 bits and half thereof becomes 96 bits. Therefore, when the blank line is transmitted by the above skip mode, the fill-bit having 67 bits is added to the blank line, and thus the data transmission time cannot be effectively shortened.